The present invention is in a method for the preparation of 1,2-dichloroethane in a reactor by absorption of chlorine in a hot, catalyst-containing, liquid stream circulating under elevated pressure, which consists of chlorinated hydrocarbons, and reacting gaseous ethylene with the chlorine dissolved in the liquid phase.
The chlorination of ethylene to 1,2-dichloroethane (EDC) is an exothermic reaction in which the released reaction heat is suitable for the production of steam at the rate of about 1 ton of stea per ton of EDC (EP-OS No. 0075 742).
Different approaches are used in the known methods (U.S. Pat. No. 4,347,039) for the recovery of this reaction heat at the highest possible EDC yield, a high ethylene conversion and a high time space yield. Thus, the reaction heat is removed either in the reactor by cooling systems installed within the reactor and by evaporative cooling of the reactor content, or outside of the reactor by product cooling and evaporation. Other differences lie in the manner in which the chlorine and ethylene reactants are fed into the reaction system, the configuration of the reaction system, in the way catalysts are used to promote the desired addition reaction to EDC and of inhibitors for reducing undesired substitution reactions, such as for example the formation of 1,1,2-trichloroethane and other higher-boiling chlorinated hydrocarbons.
In particular, those methods wherein the reaction is performed above 100.degree. C. at a pressure of more than 3 bars while the reaction system is confined in a product circuit approach having a reasonable cost of recovery of reaction heat. In such methods the release of catalyst-free product from the product circuit containing the catalyst is generally performed by flash evaporation.
As it is generally known, the efficiency of the recovery of energy, and especially the possible of producing technically useful steam, increases as the temperature level of the whole product circuit increases. On the other hand, it is also generally known that, as the temperature increases in the reactor and in the product circuit, the formation of byproducts, i.e., losses of EDC yield, due for example to substitution reactions and dehydrochlorinating cleavage of EDC, increases.
In addition to this aspect, with its corresponding influencing factors, such as catalysts and inhibitors, for example, the maximum technically achievable reaction temperature and hence also reaction pressure in ethylene chlorination is determined by the level of the chlorine input pressure. In this case the chlorine, usually coming as cell chlorine directly from a chlorine-alkali electrolysis, is fed into the reactor after compression to about 3 bar.
Directly increasing the chlorine input pressure to levels of 3 bar or more to obtain reaction temperatures of 100.degree. C. or more is technically difficult and involves an additional costly chlorine compression. The chlorine input pressure can indirectly be increased by absorption of chlorine into EDC and, as commonly practiced in the art in the case of a liquid, raising the pressure by means of a pump. In the case of EDC, this apparently technically simple and low-cost method of chlorine absorption in the cold product stream followed by pressure increase is an unsatisfactory method of increasing the energy recovery of the reaction heat, such as for example by steam generation, or for the problem of the formation of byproducts in this process.
The important disadvantages of the formerly known application of the chlorine absorption/pressure increase method are the following:
the necessary cooling of the product circuit to temperatures lower than 40.degree. C. in order to perform the desired chlorine absorption, and the subsequent heating of the chlorine-containing EDC, necessitate a considerable expenditure of energy;
due to the relatively high chlorine concentration in the EDC, of more than 8% by weight, as a result of the chlorine absorption process, it is impossible to avoid--even despite the use of inhibitors such as oxygen--the undesirable formation of byproducts, especially due to substitution reactions, on account of the need to raise the temperature of this solution to close to the reaction temperature, and the long residence time which this entails between the chlorine absorption and the start of the reaction in the reactor.
This heretofore unsatisfactory application of the chlorine absorption/pressure increase method has another disadvantageous effect. When this chlorine-containing EDC solution is heated to virtually the reaction temperature on outgassing of the chlorine from the EDC is unavoidable. A two-phase flow forms with elevated temperature and a high chlorine concentration at the phase boundary. This promotes the above-mentioned secondary reactions.
The measures disclosed heretofore for the reduction of secondary reactions at a reaction temperature above 100.degree. C. are based largely on fluid-dynamic controlling factors in addition to the advantageous use of appropriate catalysts and inhibitors in the reaction system. For example, the attempt has been made by means of a very fine and very uniform distribution of the reactants to prevent irregularities of concentration and temperatures such as can occur for example, when chlorine and ethylene bubbles meet. These include efforts to forestall contact between the gaseous reactants chlorine and ethylene, by having the chlorine dissolve preferentially in EDC and bringing this solution into a reaction with the gaseous, very finely divided ethylene.
U.S. Pat. No. 4,554,392 has recourse to this control of the reaction through the phase boundary. To do this, a reactor is selected in both cases which has the typical characteristics of a loop reactor, such as, for example, the feeding of product into the bottom part of the reactor with the flow configuration characteristic thereof, namely upward in the central inside tube and downward in the outer part, and a sufficiently great rate of circulation of the flow of, for example, 30 to 200 kilograms per square meter per second, or a velocity greater than 0.1 meter per second in the mixing zone of the inner tube.
The examples of these methods which are herein given achieve only partially their aim of controlling the reaction at the phase boundary. For example, in these embodiments of the reactor the fine distribution of the ethylene bubbles is accomplished preferentially in a nozzle provided for the purpose at the bottom of the reactor and in the inside tube of the loop reactor. In spite of the stated rates of circulation, it is not possible to avoid completely the escape of the ethylene bubbles from the reactive phase boundary of the liquid reaction phase containing catalyst and chlorine into the gas phase above it which consists of EDC in vapor form, unreacted materials, such as chlorine and especially ethylene, as well as other inert gaseous components. Especially in the case of a boiling reactor, the formation of vapor bubbles in the upper part of the liquid reaction phase causes the ethylene bubbles therein to be carried over to a greater extent and thus to be withdrawn from the desired reaction at the phase boundary surface.
The present invention is addressed to a method wherein chlorine is absorbed at a high temperature with a minimum formation of byproducts, without having to perform the above-described, disadvantageous cooling followed by heating of the product stream. A further object of the invention is to prevent any separation into a disperse gas-liquid phase and the gaseous phase situated above it at the phase boundary.